Underground water reservoir with 3X as much water as Earth's oceans found 700km deep in Earth's crust

This could be where the oceans come from

The image above, which shows all of the Earth's surface water in one place, gives you an idea of how big this new discovery is. Scientists have found, about 700 kilometers deep in the Earth's crust, a very large reservoir that holds about three times the water volume that can be found in the ocean. The water is trapped in a type of blue rock called ringwoodite, a "high-pressure polymorph of olivine (a magnesium iron silicate) that is formed at high temperatures and pressures". Here's what it looks like:

This discovery should give us new clues about the origin of the Earth's oceans. Some scientists theorize that most of the water came from comets that collided with Earth, but this new discovery could bolster the theory that claims that the water came from the planet's interior.

The cooling down of the primordial world to the point where the outgassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water.

Comets, trans-Neptunian objects or water-rich meteoroids (protoplanets) from the outer reaches of the main asteroid belt colliding with the Earth may have brought water to the world's oceans. Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on the Earth.

Planetesimals heated by the decay of aluminium. This could cause water to rise to the surface

The water may also have come from volcanism. The act of water vapor coming from volcanic eruptions then condensing and forming rain.

How can we know what's this deep inside the Earth? It's possible, with seismometers, to study the seismic waves created by earthquakes, and with the right algorithms, it is possible to determine what materials those waves passed through. Do that for hundreds of earthquakes from thousands of different seismometers, and you start to get a pretty good picture...